An article, "Picosecond Noninvasive Optical Detection Of Internal Electrical Signals In Flip-Chip-Mounted Silicon Integrated Circuits", IBM J. Res. Develop., Vol. 34, No. 2/3 (1990), pages 162-172 discloses a laser measuring method for sensing the charge carrier density in the inside of a component of microelectronics. It is based on what is referred to as a "backside-probing" technique, wherein the laser radiation is focused into the plane of the voltage-carrying components from the backside of the component using a conventional microscope objective. Since the irradiation occurs through the substrate that is approximately 0.4 mm thick, a pronounced spherical aberration arises that limits the spatial resolution to 2 through 4 .mu.m. It has therefore been proposed to diminish the spherical aberration by grinding the substrate to such an extent that a spatial resolution lying in the sub-micrometer range is achieved with a conventional microscope objective at a wavelength of .lambda.=1.3 .mu.m. Grinding down the substrate to fractions of a millimeter, however, jeopardizes the mechanical stability of the component. Moreover, it is not assured that this type of preparation has no influence on the electrical functioning of the component.
What is referred to as a Luneburg lens belongs to the class of absolute optical instruments having perfect geometrical-optical imaging. It is composed of a nonhomogeneous sphere having a radius R whose refractive index n is a function of the distance r from the center of the sphere. When the refractive index n obeys the relationship n (r)=(2-r.sup.2 /R.sup.2).sup.1/2, then every parallel beam incident from an arbitrary spatial direction is united in an ideal focus on the sphere surface. The Luneburg lens serves as antenna in microwave technology, whereby this antenna is then composed of a plurality of dielectric spherical shells having different but respectively constant refractive indices.
A Luneburg lens composed only of an inner sphere and of an outer sphere is disclosed in the references of J. Appl. Phys. 32 (1961) page 2051 and R. C. Hansen, Editor, "Microwave Scanning Antennas", Academic Press, New York (1965) pages 214-218. This lens has an extremely low spherical aberration with slight zonal aberration, so that it is employable up to an incident height of h=0.95 r.sub.1 (where r.sub.1 is the radius of the outer sphere).